The present invention relates to a flexible base plate for printed circuit board and a method for the preparation thereof. More particularly, the present invention relates to a flexible base plate for printed circuit board composed of a film or sheet of a plastic resin with flexibility and a metal foil firmly bonded to the plastic film or sheet with an adhesive.
One of the problems in the flexible base plates for printed circuit board of the above described type is the strength of adhesive bonding between the film- or sheet-like base of a plastic resin and the metal foil. Several methods have been proposed for the improvement of the adhesive bonding in the prior art in which the surface of the plastic base is subjected to certain treatments prior to adhesive bonding with the metal foil both by a chemical and by a physical means including a treatment with an alkali such as sodium hydroxide or chromic acid mixture, treatment with corona discharge and mechanical treatment for matting of the surface by sand blasting or grinding with a sand cloth.
Each of these prior art methods has disadvantages and problems. For example, the methods by the chemical means have problems that the composition or nature of the treatment solution, e.g. an alkali solution or chromic acid mixture, gradually changes in the course of repeated use of the solution to cause non-uniformity in the effectiveness of the treatment presenting a difficult problem in the quality control of the products. In addition, disposal of spent solutions requires elaborate and expensive facilities since otherwise a serious problem of environmental pollution is unavoidable. The method of treatment with corona discharge is, although this method was considered promising in the early stage of its debut, not practically undertaken for the preparation of flexible bases of printed circuit board due to the gradual loss of the effect on the improvement of adhesion along with the disappearance of the elecric charge once accumulated on the surface.
The mechanical matting method of the surface of the plastic base by sand blasting or grinding with a sand cloth is, on the other hand, free from the above described problems in the other methods but the method has its own disadvantages that the mechanical strength of the plastic base may possibly be decreased by the mechanical working in addition to a problem of troubles in the succeeding process of the adhesive bonding of a metal foil and preparation of a printed circuit board caused by the particles of the sanding or grinding powder such as silicon carbide eventually left on the surface of the plastic base after sanding or grinding. This problem can be obviated only by carefully repeating washing or cleaning to remove the abrasive particles from the surface of the plastic base.
Beside the above described problems inherent to the respective methods for the surface treatment of the plastic base, it is of course essential that the surface treating method has no adverse influences on the properties of the plastic base per se for the printed circuit board such as flexibility, heat resistance in view of the possibility of contacting with molten solder alloy, resistance against chemicals including solvents, acids and alkalis and flame retardancy. Therefore, none of the prior art methods is quite satisfactory from the standpoint of practical application.
Taking the flexible bases of a copper-foiled polyimide resin film or sheet as an example of the base material of printed circuit boards currently under wide practical use, an adhesive bonding strength or peeling resistance of 1.3 kg/cm can be usually obtained between the polyimide resin base and the copper foil when an adhesive of an epoxy-modified nylon resin is used on the surface of the resin base subjected to the sanding treatment in advance. This adhesion strength is, however, not sufficiently high in consideration of the recent trend toward higher and higher density of the printed circuit wiring in compliance with the demand for more and more compact design of electronic instruments including ICs and LSIs with increased density of integration. For example, the width of the copper foil wiring on a printed circuit board is required to be 100 .mu.m or smaller so that the above mentioned peeling resistance of 1.3 kg/cm is only 13 g for each copper foil wiring of 100 .mu.m width. Therefore, such a printed circuit board causes troubles when used in an electronic instrument under vibration or bending as in a printer head.